Education Standards
Electromagnetic waves
Overview
An introduction to EMW for FYUGP students of RU
Introduction
Electromagnetic waves are changing magnetic and electric fields. An electric field is produced by a charged particle. A force is exerted by this electric field on other charged particles. The Magnetic field is produced by a moving charged particle. A force is exerted by this magnetic field on other moving particles. The electromagnetic field is produced by an accelerating charged particle. If the frequency of oscillation of the charged particle is f, then it produces an electromagnetic wave with frequency f. Electromagnetic waves transfer energy through space.
About EMW
- Electromagnetic waves are nothing but changing magnetic and electric fields. These waves are solutions of Maxwell’s equations, which are the fundamental equations of electrodynamics.
- Wave equation governing the electromagnetic field in a uniform linear medium with zero charge density: ∇2E-μσ∂E∂t-εμ∂2E∂t2=0 and ∇2H-μσ∂H∂t-εμ∂2H∂t2=0
- The wave equation is equally true for the conducting and non-conducting medium.
- A plane wave is a wave whose amplitude remains the same at any point of the space in a plane perpendicular to a direction.
- Electromagnetic wave propagates in free space with a speed equal to that of light in free space.
- Electromagnetic field vectors E and H are perpendicular to the direction of propagation of wave. i.e. electromagnetic waves are transverse in character.
- Field vectors are mutually perpendicular to each other and, also perpendicular to the direction of propagation of wave.
- The field vectors propagate in phase in free space.
- The electromagnetic energy density is equally divided into electrostatic energy density and magnetostatic energy density.
- An isotropic medium is the one which has same properties in all directions.
- Electromagnetic wave propagates in non-conducting or dielectric medium with a speed less than that of electromagnetic wave in free space.
- Electromagnetic field vectors E and H are perpendicular to the direction of propagation of wave. i.e. electromagnetic waves in isotropic dielectric medium are transverse in character.
- Field vectors are mutually perpendicular to each other and, also perpendicular to the direction of propagation of wave.
- Field vectors experience an impedance of equal to that of wave in free space times μrϵr while propagating through the free space. Further, since the wave impedance value is real and positive, it means that the field vectors propagate in phase in dielectric space.
- Poynting vector is directed along the direction of propagation of wave i.e. energy flow in a plane electromagnetic wave in free space is along the direction of wave. Also, the Poynting vector in a dielectric medium is ϵrμrtimes that in free space. The electromagnetic energy density is equally divided into electrostatic energy density and magnetostatic energy density.
- The field vectors are spatially attenuated while propagating through a conducting medium and the attenuation being e-βn.r. The quantity β measures the attenuation and is known as absorption co-efficient. Greater the value of β, greater is the absorption.
- Skin depth or penetration depth (δ) is defined as that depth inside the conducting medium at which wave amplitude falls to 37% its amplitude at the surface of conductor.
- Skin depth decreases with the frequency of electromagnetic wave. At very high frequency, electromagnetic waves do not penetrate much into the conductor, rather flows along the surface of it. Skin depth helps to determine the thickness of conducting sheet used to carry high frequency electromagnetic wave.
- Electromagnetic waves have the usual transverse character in conducting medium too. The ratio of field vectors is a complex number, hence E and H are not in phase.
- The energy density of the wave is divided unequally among the electrostatic field and magnetostatic field, magnetostatic energy density is greater than that of electrostatic field.
- Electromagnetic wave travelling from rarer to denser medium, refracted wave is deviated towards the normal.
- An electromagnetic wave reflected from a denser medium suffers a phase change of π radians.
An OER video
MULTIPLE CHOICE QUESTIONS Self-assess your understanding
1. Free space offers an impedance of ___________ ohm to the flow of electromagnetic wave
a) 300 b) 376.6 c) 352.6 d) 400
2. The ratio of electrostatic energy density to the magnetostatic energy density for a free space propagation of electromagnetic wave is
a) 0.1 b) 2 c) 0.5 d) 1
3.At the polarizing angle, θp the radiation with its electric vector perpendicular to the plane of incidence is reflected as
a) an elliptically polarized beam in the plane of incidence
b) a plane polarized beam in the plane of incidence
c) a plane polarized beam in the plane perpendicular to plane of incidence
d) a circularly polarized beam in the plane of incidence
4. Electromagnetic field is produced by
a) an accelerating/deaccelerating charged particle
b) a stationary charged particle
c) a charge particle moving with constant speed
d) None of these
5. Energy of electromagnetic wave is due to
a) the wavelength b) the frequency
c) the electric and magnetic field d) None of these
6. In free space electromagnetic waves travel with
a) Speed of light b) Speed of sound
c) Speed of supersonic d) Speed of ether
ANSWER KEY
1-b
2-d
3-c
4-a
5-c
6-a